The present invention relates to controlling the focus of an optical reading/reproducing apparatus such as a compact disc (CD) player, and more particularly to an astigmatism method using a half-mirror.
Systems for detecting the focal point of an optical reading/reproducing apparatus can be represented by astigmatism methods which employ a half-mirror as disclosed in Japanese Provisional Publication No. 57013/1981 and Japanese Provisional Publication No. 167863/1984. According to the conventional astigmatism methods, in general, the direction of focal line of light spot on the light detector is in many cases tilted by 45 degrees with respect to a track direction (hereinafter referred to as track) which is formed on the disc surface to store the data as shown in FIG. 1. That is, FIG. 1 illustrates the conventional arrangement of light detectors 10 and a focus signal detector circuit in an astigmatism system for detecting focus. In this system, the direction of focal line (indicated by dotted lines in FIG. 1) of a light spot is tilted by 45 degrees with respect to the track direction 14 on the light detector, and a dividing line (hereinafter referred to as divide-into-four line) for dividing the light detector, 10 into four regions is arranged to be zero degree with respect to the track direction.
In the case of the half-mirror system, however, if the inclination of the direction of the focal line is set at 45 degrees, the axis of light emission of the laser diode defines an angle of 45 degrees with respect to a direction in which a pickup unit moves. Furthermore, if the axis of an optical detector system is horizontally arranged with respect to the disc using a reflecting mirror or the like in order to reduce the thickness of the pickup unit, the structure becomes so complex that it is made very difficult to mount optical parts and the mold a casing that contains the optical parts.